Compositions that produce a color change upon exposure to energy in the form of light or heat are of great interest in generating images on a variety of substrates. For example, indicia are frequently printed onto optical discs, product packaging, adhesive labels, or other substrates to provide a decorative appearance, to provide information and/or to aid in identification of the item or provide other content.
Traditionally, the production of images on paper-based or polymer-based substrates has been by way of screen, flexographic, offset, lithographic printing or other known printing methods, frequently performed at a converting facility. In the packaging industry the term “converter” refers to a company that converts raw packaging material (e.g., flexible polymers, paper, and cardboard) into useable packaging products by printing upon the material, cutting, laminating, and the like. From the converter, the packaging product, usually in the form of a roll of flexible film or paper products, is then sent to packaging houses where packaging equipment such as a vertical or horizontal form feed and seal equipment is used for packaging the products and stamping them with a tracking or ID code. In many processes, it is important that the packagers provide for the continuous operation of date-code or other package marking equipment.
Time and date stamps, common to a wide variety of commercial packages worldwide, are commonly applied primarily by thermal transfer ribbon, continuous inkjet, and laser printing processes. Specific to laser marking, the laser, using high power (6 Watts or higher) has to imprint or burn in an image by ablation. Ablation is a process in which the substrate, depending on its nature, is melted, displaced, or disintegrated by the energy of the laser beam. This process is often accompanied by smoke and particulates, which can be an undesirable side effect for the packager. Air handling and purification equipment is often used to keep the area clean of particulates and smoke that might have an adverse effect on the packaged product or the seal integrity of the packaging itself. Additionally, the use of lasers for marking is limited to relatively thick substrates (greater than 0.25 mm) where the chance of punch-through is minimal as compared to a flexible packaging film where the web thickness is usually less than 0.150 mm.
Where continuous inkjet or thermal transfer ribbons are used, there is also a need to have consumables on the premises. In such processes, changeovers for replenishment of the consumables can result in periodic down time.
One challenge that image-forming compositions present is sensitivity to heat and/or pressure. This is a particular problem in the realm of industrial packaging, such as cardboard boxes, polymer bags, and the like. During production, industrial packaging materials can be subjected to high pressure from rollers, cutters and other production equipment, and also to high temperatures, such as from lamination and sealing processes. For example, co-extrusion laminating processes, in which a tie layer is extruded by melting a thin layer of polymer between two flexible films, can reach temperatures as high as 120° C., and impose pressures up to about 1000 psi.
When subjected to elevated temperature and/or pressure, many image-forming compositions prematurely darken, reducing or eliminating their ability for later development to produce a desired image. Some image-forming compositions darken at temperatures as low as 60° to 75° C., and most will darken at relatively low temperatures when subjected to elevated pressure, depending upon the pressure level.
Thus, there is a need in the industry for compositions and methods that may overcome one or more of the deficiencies and/or inadequacies described herein.